[4.3] Tinymce Source Code Editor shortcut (#39644)

This simple PR adds a keyboard shortcut to open the source code editor

Author: brianteeman

build/media_source/plg_editors_tinymce/js/plugins/highlighter/plugin.es6.js

@@ -94,4 +94,5 @@ window.tinymce.PluginManager.add('highlightPlus', (editor, url) => {
     onAction: showSourceEditor,
     context: 'tools',
   });
+  editor.addShortcut('Alt+U', 'Opens the code editor', showSourceEditor);
 });